The present invention relates to a method for improving a property of a weld of an austenitic stainless steel as base metal, and more particularly to a treatment method for improving the weld of an assembled structure in which it is impossible to perform a solution heat-treatment that would be usually performed for preventing stress corrosion cracking (SCC) at the weld of the stainless steel, and a melting treatment instead of the solution heat-treatment may be performed only from one side of the weld. A high corrosion-resistance is needed for ensuring reliability and a long service life in instruments and parts used in high temperature and high pressure water containing dissolved oxygen, particularly, instruments for a boiling-water reactor (BWR). The present invention meets this requirement.
An intergranular stress corrosion cracking (SCC) (or intercrystalline fracture) is likely to be generated in a weld, that has been affected by a welding heat, of a primary cooling water pipe (made of JIS SUS304) in a boiling-water reactor (BWR). As shown in FIG. 3, the SCC is generated in the overlapped portion (4) of high tensile stress region (1) exceeding 0.2% proof stress, chromium-depleted layer (2) (sensitive region) generated along crystalline intergranular surfaces by the welding heat affect and corrosion environment (3) such as dissolved oxygen and the like.
In accordance with a conventional welding method in which the pipe made of JIS SUS 304 (austenitic stainless steel) is cooled down to room temperature, a high residual welding tensile stress up to several times of 10 kg/mm.sup.2 is generated in an outer surface of the pipe by welding (refer to the welding portion 6) as shown by curve 8 in FIG. 2, in which TS denotes a tensile stress region and CS denotes a compression stress region. A line 9 denotes a reference level of 10 kg/mm. The chromium-depleted layer is formed in the heat-affected portion (or zone) 7 in the vicinity of the weld in a base metal 5a. Thus, if the corrosive fluid is brought into contact with both inner and outer surfaces of the base metal having the chromium-depleted layer and the high residual tensile stress, there is a large fear that the SCC would be generated in the heat-affected portion of the base metal.
Japanese Patent Examined Publication No. 59-21711 discloses one example of the method for coping with the SCC problem. In the method proposed in that publication, an corrosion resistant material containing delta (.delta.) ferrite is overlaid by welding on surfaces of a plurality of stainless steel members to be weldingly bonded and brought into contact with the corrosive fluid in the vicinity of the weld. Subsequently, a melting treatment is performed onto the surfaces in contact with the corrosive fluid at toes of the overlaid layer with an input heat of 5 KJ/cm or less. Thereafter, the welding joint portion of the members made of stainless steel is welded. The purpose of this melting treatment is to eliminate the chromium-depleted layer to be generated in the heat-affected portion of the base metal by the overlay welding of the corrosion resistant material and to generate a formation that includes .delta. ferrite and is superior in corrosion resistance property. Also, a method set forth in Japanese Patent Unexamined Publication No. 53-56134 is similar to that disclosed in the above-described Publication No. 59-21711 in that the melting treatment is performed in the vicinity of the welding joint portion of the stainless steel prior to the welding work. Namely, the method is that the melting treatment is attained by imparting a heat energy such as arc and plasma to a surface layer to be affected by the welding work prior to the welding work of the members made of austenitic stainless steel. According to this method, the .delta. ferrite is generated when the molten portion through the melting treatment is solidified. Even if the micro structure obtained according to this method is affected by the welding heat, there is no sensitive region due to precipitation of carbides in the crystalline intergranular surfaces in the same manner as the application of the corrosion resistant material.
Another method for coping with the SCC is disclosed in Japanese Patent Examined Publication No. 60-45033. In that method, a corrosion resistant material containing 6 ferrite is overlaid by welding on surfaces of the welding members in contact with the corrosive fluid, and thereafter, a material is overlaid by welding on the surfaces opposite to toes of the previously overlaid layer (or bead), while cooling the surface of the latter layer side. The purpose of the overlay welding while cooling is to improve the residual stress in the chromium-depleted region generated in the heat-affected portion of the base metal by the first (or primary) application of the corrosion resistant material.
The well-known methods are effective against the SCC only with respect to the surfaces where the SCC countermeasure has been effected. However, in the conventional methods, there is no consideration of the surface opposite to the treated surface. Accordingly, both side surfaces of the joints must be subjected to the SCC countermeasure in the case where both surfaces of the welding joints are brought into contact with the corrosive fluid. This means that the prior art is only effective in the case where the welded joint is brought into contact with a corrosive fluid on both of opposed surfaces and it is impossible to apply the melting treatment on both the side of the welding joint.
On the other hand, the above-described Publication No. 53-56134 and Japanese Patent Unexamined Publication No. 63-177972 (which generally relates to a cold working) shows a method in which in order to prevent the SCC generation in the heat-affected portion of the welding joint made of austenitic stainless steel, prior to the welding work, the portion of the base metal to be affected by the welding heat is molten by the application of the heat energy. However, this method is not applied to the welded joint that has been already formed.
The present invention has been made under such a technical background.